Biological racism refers to the idea that the races are biologically different and that some races are superior to others in intelligence and moral characteristics.

Biological racism was most popular in the United States in the 1920s when IQ (intelligence quotient) tests were first developed. It also took the form of anti-immigrant reactions to the influx of non-Protestant immigrants from southern, central, and eastern European countries—the influx that peaked in the first decade of the 20th century. Biological racism thus provided the intellectual justification for the Immigration Act of 1924, which drastically restricted immigration from these regions of Europe, as well as for the eugenics movement and antimiscegenation laws. Moreover, it also reinforced popular perceptions within U.S. white society regarding the inferiority of African Americans, Native Americans, and Latinos.

A foundational tenet of biological racism is the idea of hereditarianism. Hereditarianism emerged in the late 19th and early 20th centuries, when Charles Darwin's ideas of evolution dominated much of the intellectual world. Darwin's theory of evolution posited that beneficial traits evolved through natural selection and that only the fittest species survive. This idea was later applied to human beings and called Social Darwinism. Social Darwinism held that people who were successful were fundamentally more "fit" than people who were not successful; that is, nature rewarded those who were biologically superior.

Social Darwinists further argued that biologically inferior people passed that inferiority to their children. Similarly, they thought that biological superiority was also inherited. Based on this idea, Hereditarians of the late 19th and early 20th centuries supported both limiting the reproduction of those thought to be biologically inferior and antimiscegenation laws that would prevent the race mixing they feared would degrade the white race. The attempt to limit reproduction of people deemed unfit through public policy is known as eugenics.

Eugenicist Madison Grant advocated the idea of biological differences among racial groups. In his book, The Passing of the Great Race (1916), Grant argued that the races were in fact subspecies of man and that the "Nordic" race—those with light hair and eyes—was superior to "Negroid" and "Mongoloid" races. Grant argued that there were clear and immutable differences among the races that were reflected in physical attributes such as height, skin and eye color, and skull shape. He further argued that these physical characteristics were associated with spiritual, intellectual, and temperamental traits. He argued that the "white race" was in jeopardy of being dominated by what he considered to be the inferior races.

In the early 20th century, Lewis Terman, a Stanford University psychologist, popularized widespread IQ testing in the United States. Following the eugenicist movement of the time, Terman worried that individuals with low IQ scores could lower the quality of American "stock"—the biological fitness of the American population. As a result, he was a strong supporter of immigration restrictions and of sterilizing men and women with low IQ scores or other "undesirable" traits, such as mental illness. Like many eugenicists of the time, Terman believed that intelligence, morality, and other human traits were linked and heritable; therefore, he viewed limiting or eliminating the reproduction of people with those characteristics as sound goal for social policy.

Persuaded by the hereditary argument of Terman and other psychologists, Congress passed the National Origins Act of 1924. A decade after the immigration act, the emergence of fascism affected much of central Europe. Jewish refugees—one of the restricted groups—were turned away from the United States even though the total quota of immigrants had not been met. It has been estimated that 6 million southern, central, and eastern Europeans were barred from admission to the United States in the 1930s on the basis of their nationality or "race."

Eugenics and the argument that there are significant biological differences among the "races" began to become socially and politically unacceptable after World War II. The National Socialists, or Nazis, in Germany had justified the Holocaust on alleged biological inferiorities of Jews and other groups. The Nazi policy to produce a "master race" of so-called Aryans left many people deeply wary of claims that some "races" are biologically superior to others. Biological racism, however, has since occasionally reemerged, particularly around social policies such as Head Start in the 1960s.

Some contemporary scholars continue to argue that race is a biological category and that differences in ability observed among racial groups are inborn. One of the most prominent and controversial contemporary books on race and IQ is The Bell Curve (1994) by Richard Herrnstein and Charles Murray. Herrnstein and Murray argued that social hierarchies reflect real differences in ability. In other words, smarter people are more likely to be rich and powerful. Moreover, they argue that this is not just true for individuals that some groups, on average, have lower IQs than others. Society therefore should not be surprised that some groups, notably African Americans, do not do as well economically as other groups. Herrnstein and Murray argue that there are real differences between the average cognitive abilities of Asians, whites, and blacks that account for some of the social stratification that characterizes the United States.

Critics of Herrnstein, Murray argue that their perspective ignores the well-documented effects of culture, racism, and testing bias in accounting for differences in IQ scores and social position. To such critics, much of the research on racial differences is politics disguised as science. Practitioners of this research counter that what they are doing is pure science that should not be constrained by political concerns. Being able to identify the biological basis of racial differences, however, depends on identifying the existence of distinct racial groups, which to date no one has been able to do. Race remains an important social and cultural category, but the most advanced genetic research casts doubt on the idea that it is a biological category. Thus, at this time observed differences in the IQ test scores and social status among the "races" cannot be attributed to biological differences.

The idea that there are biologically based differences among "the races" has lost credibility among academic researchers. Some researchers have argued that there is little reason to see "the races" as distinct biological groups at all. Classifications such as male and female, for example, have a biological basis. It is possible to identify an individual's sex based on his or her chromosomal makeup, if not through visual inspection. There is no comparable test for race. Biological racism is currently rejected by mainstream scientists, many of whom question whether there is any biological basis to the classification of "race" at all. In the popular debate, however, a weaker notion of biological differences among the races has persisted.